Some vehicles are hard/slow to fill with fuel. The general indication is that when re-fuelling, the gas pump shuts off repetitively after only adding a minor volume of gas. The fuel seems to back up into the fill tube, causing the automatic shut off on the pump to activate. The fuel gradually subsides in the filler neck, and some more gas can be added before shutting off again.
There are two general possibilities which would cause the issue: a restriction directly preventing fuel from entering the tank, or a restriction preventing vapours from exiting the tank and thus not allow fuel to enter. All cars built from 1999 onward have had included an onboard fuelling vapour recovery system. Simplified this means that fumes from the entire fuel system are no longer vented to the outdoors, but are instead re-cycled through the vehicle air intake system.
Things that can prevent proper venting include blockages anywhere in the onboard vapour recovery system such as for instance a blocked leak detection pump outlet, restricted charcoal canister filter, or a restricted charcoal canister itself by liquid fuel in canister.
One possible cause of blockage in the venting system is from fuel entering the vapour, vent lines and directly into the charcoal canister. The liquid gasoline gets absorbed by the charcoal in the canister, causing it to swell and it blocks off the vapour vent lines. A clogged canister filter may prevent one from filling up with gas, since the gas tank contains both fuel and air. As the tank is filled up, the gasoline displaces the air, and the air has to escape somehow. But the air that is displaced is full of fuel vapour.
Being harmful to the atmosphere, fuel vapour can't just be vented directly into the air, so the vapour recovery system is set up to capture these vapours through a series of vent lines that lead to a box full of charcoal, which absorbs the gas vapours. When the gas-soaked charcoal swells and blocks off the vent lines, it is preventing the air from escaping the tank. In order to put gas in, the air has to have some way of getting out, and the only way for it to get out is through the vent lines. Common solutions involve replacing one of the components in the fuel vapour recovery system. (one way valve in the tank, vent valve in the tank, liquid-vapour separator near the tank, carbon canister or canister close valve.) The most common first choice for repair is the canister, but the other components may well be plugged.
Under specific circumstances a fuel vent valve might leak. Vehicles with an internal combustion engine are generally equipped with a system that checks whether or not the venting systems leak. They may also be equipped with an evaporative system that purges the canister during driving. These checks include pressurizing the system and the purging of the canister is normally performed when the engine is running If a leakage diagnose check is performed when the fuel tank is full and/or the vehicle is tilted when not full, liquid fuel might pass the fuel vent valve and flow on to clog up the canister.
FIG. 1 shows a standard fuel vent valve 1 in a closed position where a float 2 with a sealing 3 is lifted up to close the opening 4. A part 5 of the valve housing has openings to allow for liquid to pass through. During a “leak” check or when purging the canister the float 2 might be made to oscillate and if the valve 1 at the same time with a liquid level relative to vent valve shut off point, e.g., is being on the border to close (seal), the valve might leak and let liquid fuel into the vent system 6 and further on to the canister. The canister (not shown) can handle vapour but not liquid.